Optical fiber has acquired an increasingly important role in the field of telecommunications, frequently replacing existing copper wires. This trend has had a significant impact in all areas of telecommunications, greatly increasing the amount of data that is transmitted. Further increase in the use of optical fiber is foreseen, especially in metro and fiber-to-the-home applications, as local fiber networks are pushed to deliver an ever-increasing volume of audio, video, and data signals to residential and commercial customers. In addition, use of fiber in home and commercial premise networks for internal data, audio, and video communications has begun, and is expected to increase.
Optical fiber is typically made of glass, and usually has a polymeric primary coating and a polymeric secondary coating. The primary coating (also known as an inner primary coating), is typically applied directly to the glass fiber and, when cured, forms a soft, elastic, compliant material encapsulating the glass fiber. The primary coating serves as a buffer to cushion and protect the glass fiber during bending, cabling, or spooling. The secondary coating (also known as an outer primary coating) is applied over the primary coating, and functions as a tough, protective, outer layer that prevents damage to the glass fiber during processing, handling, and use.
Secondary coatings conventionally used in optical fibers are typically crosslinked polymers formed by curing a mixture of an oligomer (e.g. a urethane (meth)acrylate) and at least one monomer (e.g. a (meth)acrylate monomer). Generally, increasing the modulus of a urethane/acrylate oligomer based on crosslinked coating results in an accompanying increase in tensile strength and a decrease in elongation at break (McConnell et al., ACS Symp. Ser. 417:272-283 (1990)). This generally increases the brittleness of these materials, resulting in coatings considered to have poor toughness. Rigid or multi-functional oligomeric coating additives that could increase modulus, while still maintaining high values of both tensile strength and elongation at break, would be advantageous.
Extensive literature exists on the toughening of polymeric materials (Polymer Toughening, Arends, ed., Marcel Dekker (1996) and Thermosetting Polymers, Pascault et al., eds., Marcel Dekker (2002)). Many of the toughening concepts applied to thermoplastics have also been applied to crosslinked materials. Most of this work has been done on thermoset materials, particularly epoxy coatings (see Calzia et al., Antec Proceedings 2258-2268 (2004) and references cited therein). A number of specific approaches have been taken to modify crosslinked epoxy networks, including: (i) the uniform dispersion of both reactive and non-reactive, soft, rubber-like materials throughout the coating; (ii) the dispersion of hard, reinforcing materials (such as silica, carbon black, clays, and carbon fibers) throughout the coating; and (iii) the dispersion of high molecular weight polymeric additives. Most of these toughening agents are believed to be uniformly phase separated throughout the material; they function by absorbing and dissipating external energy to mitigate crack initiation or internal energy and to slow propagation of growing cracks in the crosslinked networks. In addition, epoxy coatings have been prepared from epoxide and amine components containing chemical structures that are tied into the network structure as a means to dissipate energy (via bond shifts or rotations) and provide toughening (see Lesser et al., J. Poly. Sci.: Part B: Polymer Physics 42:2050-2056 (2004) and references cited therein). In general, it is much more difficult to find examples of similar approaches to toughening in ultra-violet (“UV”) curable acrylic coatings. This may be due to the very fast curing process in the acrylic coatings compared to the epoxies, which may preclude the development of the phase separated network structural features needed for some of the above cited mechanisms to operate efficiently.
Multi-functional urethane/acrylate oligomers containing a very rigid, non-alkoxylated bisphenol A diol in their “arms” have previously been described (U.S. Pat. No. 6,862,392 to Fabian et al.). While all of these materials were found to improve the toughness of secondary fiber coatings, they were all capped with a reactive hydroxy acrylate group that ensured that the additive became chemically bonded to the polymer coating network during photocuring and also was believed to ensure that the additives were uniformly dispersed throughout the coatings. These additives had the desired effect of toughening the resulting coatings. However, these additives-due to their reactivity-rendered the coating compositions less stable to storage and, surprisingly, susceptible to premature gelation that could result in coating defects during manufacture.
Examples of other multi-functional reactive oligomeric materials with structures similar to those previously reported are described in U.S. Pat. No. 5,578,693 to Hagstrom et al., but these materials generally contain higher molecular weight flexible polyols to reduce the oligomer viscosity and to increase its utility in coatings. There is no indication that these oligomers provide enhanced coating toughness despite also being chemically bound into the coating network.
The present invention is directed to curable compositions for secondary optical fiber coatings that overcome these and other limitations in the art.